Lubricating compositions for preventing or reducing pre-ignition in an engine

ABSTRACT

Disclosed is the use of a lubricating composition including at least one base oil and at least one polyalkylene glycol (PAG) produced by polymerisation or copolymerisation of alkylene oxides including between 3 and 8 carbon atoms, at least one of which being a butylene oxide, for preventing or reducing pre-ignition in an engine, preferably in a vehicle engine.

The invention relates to the use of a lubricating composition to prevent or reduce pre-ignition in an engine. More particularly, the invention relates to the use of a lubricating composition comprising at least one base oil and at least one polyalkylene glycol (PAG) to prevent or reduce pre-ignition in an engine, preferably of a vehicle, particularly of a motor vehicle.

The present invention also relates to a method for preventing or reducing pre-ignition in an engine, preferably of a vehicle, particularly of a motor vehicle using this lubricating composition.

The present invention also relates to the use of a polyalkylene glycol (PAG) in a lubricating composition to prevent or reduce pre-ignition in an engine, preferably of a vehicle engine, particularly of a motor vehicle.

TECHNOLOGICAL BACKGROUND

Under ideal conditions, normal combustion in a spark-ignition engine occurs when a fuel mixture, in particular of fuel and air, is ignited in the combustion chamber inside the cylinder through the production of a spark produced by a spark plug. Said normal combustion is generally characterized by expansion of the flame front through the combustion chamber in orderly, controlled manner.

However, in some cases, the air/fuel mixture may be prematurely ignited via a flame source prior to firing by the spark from the spark plug, leading to a phenomenon known as pre-ignition.

Yet, it is preferable to reduce and even eliminate pre-ignition since this generally translates as the presence of a strong rise in temperatures and pressures inside the combustion chamber, thereby having a significant negative impact on the efficiency and overall performance of an engine. In addition, pre-ignition may cause significant damage to cylinders, pistons, plugs and valves in the engine and, in some cases, may even lead to engine failure even engine breakage.

Recently, Low Speed Pre-Ignition (LSPI) has been identified in particular by motor vehicle manufacturers, as being a potential problem for downsized engines. LSPI generally occurs at low speeds and under heavy loads, and can cause serious damage to pistons and/or cylinders. This phenomenon is notably described by Dahnz et al (SAE Int. J. Engines, volume 3, issue 1, 2010-01-0355).

It would therefore be desirable to have available a lubricant that is able to prevent or reduce the risk of pre-ignition, including LSPI.

Solutions consisting of reducing the calcium content or increasing the zinc dithiophosphate or molybdenum dithiocarbamate content of a lubricant have been described (Takeuchi et al, “Investigation of Engine Oil Effect on Abnormal Combustion in Turbocharged Direct Injection—Spark Ignition Engines,” SAE Int. J. Fuels Lubr. 5(3):1017-1024, 2012; Hirano et al, “Investigation of Engine Oil Effect on Abnormal Combustion in Turbocharged Direct Injection—Spark Ignition Engines (Part 2),” SAE Technical Paper 2013-01-2569, 2013). However, these solutions currently remain insufficient for a significant reduction in pre-ignition and are difficult to implement especially in countries in which a high basicity level is required for fuels. In addition, problems of lubricant stability or compatibility with post-treatment systems are associated with these solutions.

Application WO2015023559 describes a method for reducing pre-ignition via the addition to a lubricant of an additive allowing delayed ignition, said additive being selected from among organic compounds comprising at least one aromatic core However, these light organic compounds could lead an excessive increase in the volatility of the lubricant.

It is therefore one objective of the present invention to provide compounds and a lubricating composition comprising these compounds to overcome all or part of the aforementioned drawbacks.

Another objective of the present invention is to provide a lubricating composition allowing pre-ignition in an engine to be prevented or reduced, and having a formulation that is easy to implement.

Another objective of the present invention is to provide a lubricating composition allowing pre-ignition in an engine to be prevented or reduced, whilst maintaining satisfactory even improved lubricating properties.

A further objective of the present invention is to provide a lubrication method allowing pre-ignition in an engine to be prevented or reduced.

SUMMARY OF THE INVENTION

The subject of the invention is therefore the use of a lubricating composition comprising at least one base oil and at least one polyalkylene glycol (PAG) obtained by polymerising or copolymerising alkylene oxides having 3 to 8 carbon atoms, including at least one butylene oxide, to prevent or reduce pre-ignition in an engine.

Surprisingly, the applicant has found that the presence of at least one PAG obtained by polymerisation or copolymerisation of alkylene oxides having 3 to 8 carbon oxides, including at least one butylene oxide, in a lubricating composition allows the lubricating composition when used in an engine to prevent or reduce pre-ignition in the engine.

With the present invention it is therefore possible to formulate lubricating compositions having both good stability and good properties for preventing or reducing pre-ignition when used in an engine.

Advantageously, the lubricating compositions of the invention have good properties for the prevention or reduction of pre-ignition when used in an engine, without the need for association thereof with other technical solutions for the prevention or reduction of pre-ignition, and in particular technical solutions requiring a reduction in calcium content or leading to an excessive increase in volatility of the lubricating composition.

Advantageously, the lubricating compositions of the invention have good properties for pre-ignition prevention or reduction when used in an engine, and the formulation thereof is easy to implement.

In one embodiment of the invention, the PAG is a PAG of formula (A):

-   -   where:         -   Y₁ and Y₂ are each independently a hydrogen atom or             hydrocarbon group, preferably a C₁-C₃₀ alkyl or alkylphenyl             group;         -   n is an integer of 2 or higher, preferably lower than 500,             more preferably from 2 to 200, further preferably from 2 to             60 or from 5 to 30 or from 7 to 20, advantageously from 7 to             15;         -   x represents one or more integers ranging from 1 to n;         -   the groups R_(2x-1) and R_(2x) are each independently a             hydrogen atom or hydrocarbon group having 1 to 6 carbon             atoms, preferably a straight-chain or branched C₁-C₆ alkyl             group,             the sum of the number of carbon atoms of R_(2x-1) and R_(2x)             is equal to 2 for at least one value of x.

In another embodiment of the invention, the PAG is a PAG of formula (A) where:

-   -   Y₂ is a hydrogen atom;     -   R_(2x-1) and R_(2x), the same or different, are each         independently a hydrogen atom or C₁-C₂-alkyl group.

In another embodiment of the invention, the PAG is a PAG of formula (B):

-   -   where:         -   R1 is a straight-chain or branched C₅-C₁₂-alkyl group;         -   p is an integer ranging from 2 to 10, preferably from 2 to             6;         -   q is an integer ranging from 2 to 10, preferably from 2 to             6.

In another embodiment of the invention, the weight content of PAG, relative to the total weight of the lubricating composition, ranges from 5 to 50%, preferably from 10 to 40%, advantageously from 15 to 30%.

In another embodiment of the invention, the lubricating composition also comprises an additive selected from among friction modifiers, detergents, anti-wear additives, extreme-pressure additives, viscosity index improvers, dispersants, antioxidants, pour point improvers, defoamers, thickeners and mixtures thereof,

In another embodiment of the invention, the lubricating composition is used to prevent or reduce pre-ignition in an engine of a vehicle, preferably of a motor vehicle.

In another embodiment of the invention, the lubricating composition is used to prevent or reduce low speed pre-ignition (LSPI) in an engine, preferably of a vehicle, advantageously of a motor vehicle.

The invention also concerns a method for preventing or reducing pre-ignition in an engine, said method comprising at least one step to place a mechanical part of the engine in contact with a lubricating composition such as defined above.

The invention also concerns the use of a polyalkylene glycol (PAG) according to the invention in a lubricating composition comprising at least one base oil to prevent or reduce pre-ignition in an engine.

DETAILED DESCRIPTION OF THE INVENTION

The percentages given below correspond to weight percentages of active material.

The lubricating composition used in the invention comprises at least one polyalkylene glycol (PAG) obtained by polymerisation or copolymerisation of alkylene oxides having 3 to 8 carbon atoms, including at least one butylene oxide.

The PAG may be a polymer or copolymer (statistical or block) of alkylene oxides, that can particularly be prepared following methods described in application WO 2009/134716, page 2 line 26 to page 4 line 12, for example via attack by an alcohol initiator on the epoxy bond of an alkylene oxide and propagation of the reaction.

In one embodiment of the invention, the PAG is obtained by polymerisation or copolymerisation of alkylene oxides having 3 to 8 carbon atoms, including at least one butylene oxide, particularly selected from among 1,2-butylene oxide or 2,3-butylene oxide, preferably 1,2-butylene oxide.

Among these PAGs, the copolymers of butylene oxide and propylene oxide are particularly preferred.

Application WO2011/011656, paragraphs [011] to [014] describes a preparation mode, the characteristics and the properties (particularly of solubility and miscibility in base oils) of said PAG copolymers of butylene oxide and propylene oxide.

These PAGs can be prepared by reaction of one or more alcohols with a mixture of butylene oxide and propylene oxide.

Advantageously, the PAGs are prepared with a mixture of butylene oxide and propylene wherein the weight ratio between butylene oxide and propylene oxide has a value ranging from 3:1 to 1:3, preferably ranging from 3:1 to 1:1.

In one preferred embodiment of the invention, the PAGs are prepared from alcohol having 8 to 12 carbon atoms. 2-ethylhexanol and dodecanol, alone or in a mixture, and in particular dodecanol, are particularly preferred.

According to one preferred embodiment of the invention, the PAGs of the invention are such that their carbon to oxygen molar ratio is higher than 3:1, preferably ranging from de 3:1 to 6:1. In particular, this imparts properties of polarity and viscosity index to said PAGs, that are particularly suited for use in an engine oil.

In another embodiment of the invention, the PAGs are prepared with a mixture of butylene oxide and propylene oxide wherein the weight ratio between butylene oxide and propylene oxide is higher than 3:1, preferably higher than 5:1, more preferably higher than 8:1, advantageously ranging from 8:1 to 10:1.

In another embodiment of the invention, the PAG is obtained by polymerisation of butylene oxides alone, particularly selected from among 1,2-butylene oxide or 2,3-buylene oxide, preferably 1,2-butylene oxide.

In another embodiment of the invention, the PAG is a PAG of formula (A):

where:

-   -   Y₁ and Y₂ are each independently a hydrogen atom or hydrocarbon         group, preferably a C₁-C₃₀ alkyl or alkylphenyl group;     -   n is an integer of 2 or higher, preferably lower than 500, more         preferably from 2 to 200, further preferably from 2 to 60 or         from 5 to 30 or from 7 to 20, advantageously from 7 to 15;     -   x represents one or more integers ranging from 1 to n;     -   the groups R_(2x-1) and R_(2x) are each independently a hydrogen         atom or hydrocarbon group having between 1 and 6 carbon atoms,         preferably a straight-chain or branched C₁-C₆ alkyl group,         the sum of the number of carbon atoms of R_(2x-1) and R_(2x) is         equal to 2 for at least one value of x.

In one preferred embodiment, the groups R_(2x-1) and R_(2x) are each independently a straight-chain C₁-C₆ alkyl group.

In another preferred embodiment of the invention, at least one of the groups R_(2x-1) and R_(2x) is a hydrogen atom. Advantageously, R_(2x) is a hydrogen atom.

In another preferred embodiment of the invention, the sum of the number of carbon atoms of R_(2x-1) and R_(2x) has a value ranging from 1 to 6, and the sum of the number of carbon atoms of R_(2x-1) and R_(2x) is equal to 2 for at least one value of x.

In another preferred embodiment of the invention, the PAG is a PAG of formula (A) where:

-   -   Y₂ is a hydrogen atom;     -   R_(2x-1) and R_(2x), the same or different, are each         independently a hydrogen atom or     -   C₁-C₂-alkyl group.

Advantageously, the PAG is a PAG of formula (B)

where:

-   -   R1 is a straight-chain or branched C₈-C₁₂-alkyl group;     -   p is an integer ranging from 2 to 10, preferably from 2 to 6;     -   q is an integer ranging from 2 to 10, preferably from 2 to 6.

The PAGs are preferably low molecular weight PAGs characterized by kinematic viscosity, measured at 100° C. as per standard ASTM D445, ranging from 1 to 12 mm²/s, preferably from 2 to 7 mm²/s.

Preferably, the viscosity index VI of the PAGs used in the invention, measured as per standard NFT 60136, is 100 or higher, preferably 120 or higher.

Preferably, the lubricating composition used in the invention has a weight content of PAG relative to the total weight of the lubricating composition ranging from 5 to 50%, preferably from 10 to 40%, advantageously from 15 to 30%.

As examples of PAGS able to be used in the lubricating composition used in the invention, mention can be made of the PAGs described in documents WO 2012/070007 or WO 2013/164457.

The lubricating composition used in the invention also comprises at least one base oil.

In general, the lubricating composition used in the invention may comprise any type of mineral, synthetic or natural, animal or vegetable lubricating base oil known to persons skilled in the art.

The base oils used in the lubricating compositions of the invention may be oils of mineral or synthetic origin belonging to Groups I to V in the classes defined by the API classification (or equivalents thereof in the ATIEL CLASSIFICATION) (Table A), or mixtures thereof.

TABLE A Saturates Viscosity index content Sulfur content (VI) Group I <90% >0.03% 80 ≤ VI < 120 Mineral oils Group II ≥90% ≤0.03% 80 ≤ VI < 120 Hydrocracked oils Group III ≥90% ≤0.03% ≥120 Hydrocracked or hydro-isomerised oils Group IV Polyalphaolefins (PAOs) Group V Esters and other bases not included in Groups I to IV

The mineral base oils of the invention include all types of base oils obtained by atmospheric and vacuum distillation of crude oil, followed by refining operations such as solvent extraction, deasphalting, solvent dewaxing, hydro-treatment, hydrocracking, hydro-isomerisation and hydrofinishing.

Mixtures of synthetic and mineral oils can also be utilised.

In general, there is no limitation as to the use of different lubricating bases to produce the lubricating compositions used in the invention, other than that they must have properties, particularly of viscosity, viscosity index, sulfur content, resistance to oxidization, adapted for use in engines or for vehicle transmissions.

The base oils of the lubricating compositions used in the invention may also be selected from among synthetic oils such as some esters of carboxylic acid and alcohols, and from among polyalphaolefins. The polyalphaolefins used as base oils are obtained for example from monomers having 4 to 32 carbon atoms e.g. from octene or decene, and having viscosity at 100° C. of between 1.5 and 15 mm²·s⁻¹ as per standard ASTM D445. Their molecular weight average is generally between 250 and 3 000 as per standard ASTM D5296.

Preferably, the base oils of the present invention are selected from among the above base oils having an aromatic content of between 0 and 45%, preferably between 0 and 30%. The aromatic content of the oils is measured in accordance with the UV Burdett method.

Advantageously, the lubricating composition used in the invention comprises at least 50% by weight of base oils relative to the total weight of the composition.

More advantageously, the lubricating composition used in the invention comprises at least 60% by weight, even at least 70% by weight of base oils relative to the total weight of the composition.

Further advantageously, the lubricating composition used in the invention comprises from 60 to 99.5% by weight of base oils, preferably from 70 to 99.5% by weight of base oils relative to the total weight of the composition.

Numerous additives can be used for this lubricating composition used in the invention.

The preferred additives for the lubricating composition used in the invention are selected from among friction modifiers, detergents, anti-wear additives, extreme-pressure additives, viscosity index improvers, dispersants, antioxidants, pour point improvers, defoamers, thickeners and mixtures thereof.

Preferably, the lubricating composition used in the invention comprises at least one anti-wear additive, at least one extreme-pressure additive or mixtures thereof.

The anti-wear additives and extreme-pressure additives protect friction surfaces by forming a protective film adsorbed on these surfaces.

There are a large variety of anti-wear additives. Preferably, for the lubricating composition of the invention, the anti-wear additives are selected from among phospho-sulfurized additives such as metal alkylthiophosphates, in particular zinc alkylthiophosphates, and more specifically zinc dialkyldithiophosphates or ZnDTPs. The preferred compounds have the formula Zn((SP(S)(OR²)(OR³))₂, where R² and R³, the same or different, are each independently an alkyl group, preferably an alkyl group having 1 to 18 carbon atoms.

Amine phosphates are also anti-wear additives that can be used in the lubricating composition of the invention. However, the phosphorus contributed by these additives may act as poison for catalytic systems of motor vehicles since these additives generate ash. These effects can be minimised by partly substituting amine phosphates by additives that do not contain phosphorus such as polysulfides for example, in particular sulfurized olefins.

Advantageously the lubricating composition of the invention may comprise from 0.01 to 6% by weight, preferably from 0.05 to 4% by weight, more preferably from 0.1 to 2% by weight of anti-wear additives and extreme-pressure additives relative to the total weight of the lubricating composition.

Advantageously, the lubricating composition of the invention may comprise at least one friction modifying additive. The friction modifying additive can be selected from among a compound providing metal elements and an ash-free compound. Among the compounds providing metal elements, mention can be made of transition metal complexes such as Mo, Sb, Sn, Fe, Cu, Zn, the ligands of which may be hydrocarbon compounds comprising atoms of oxygen, nitrogen, sulfur or phosphorus. The ash-free friction modifying additives are generally or organic origin and can be selected from among the monoesters of fatty acids and polyols, alkoxylated amines, alkoxylated fatty amines, fatty epoxides, borate fatty epoxides; fatty amines or fatty acid glycerol esters. According to the invention the fatty compounds comprise at least one hydrocarbon group having 10 to 24 carbon atoms.

Advantageously, the lubricating composition of the invention may comprise from 0.01 to 2% by weight, or from 0.01 to 5% by weight, preferably from 0.1 to 1.5% by weight or 0.1 to 2% by weight of friction modifying additive relative to the total weight of the lubricating composition.

Advantageously, the lubricating composition of the invention may comprise at least one antioxidant additive.

An antioxidant additive generally allows delayed degradation of the lubricating composition in use. This degradation may notably translate as the formation of deposits, as the presence of sludge or as an increase in viscosity of the lubricating composition.

Antioxidant additives particularly act as radical inhibitors or hydroperoxide decomposers. Among the antioxidant additives frequently employed, mention can be made of antioxidant additives of phenolic type, antioxidant additives of amino type, phosphor-sulfurized antioxidant additives. Some of these antioxidant additives e.g. phospho-sulfurized antioxidant additives may generate ash.

Phenolic antioxidant additives may be ash-free or may be in the form of neutral or basic metal salts. Antioxidant additives can be selected in particular from among sterically hindered phenols, sterically hindered phenol esters and sterically hindered phenols comprising a thioether bridge, diphenylamines, diphenylamines substituted by at least one C₁-C₁₂ alkyl group, N,N′-dialkyl-aryl-diamines, and mixtures thereof.

Preferably, according to the invention, the sterically hindered phenols are selected from among compounds comprising a phenol group wherein at least one vicinal carbon of the carbon carrying the alcohol function is substituted by at least one C₁-C₁₀ alkyl group, preferably a C₁-C₆ alkyl group, preferably a C₄ alkyl group, preferably by the tert-butyl group.

Amino compounds are another class of antioxidant additives that can be used, optionally in combination with phenolic antioxidant additives. Examples of amino compounds are the aromatic amines e.g. the aromatic amines of formula NR⁴R⁵R⁶ where R⁴ is an aliphatic group or aromatic group, optionally substituted, R⁵ is an aromatic group, optionally substituted, R⁶ is a hydrogen atom, an alkyl group, an aryl group or a group of formula R⁷S(O)_(z)R⁸ where R⁷ is an alkylene group or alkenylene group, R⁸ is an alkyl group, an alkenyl group or aryl group and z is 0, 1 or 2.

Sulfurized alkyl phenols or the alkaline or alkaline-earth metal salts thereof can also be used as antioxidant additives.

Another class of antioxidant additives is that of copper compounds e.g. copper thio- or dithio-phosphates, copper and carboxylic acid salts, copper dithiocarbamates, sulfonates, phenates and acetylacetonates. Copper I and II salts, the salts of succinic acid or anhydride can also be used.

The lubricating composition of the invention may contain any type of antioxidant additives known to persons skilled in the art.

Advantageously, the lubricating composition comprises at least one ash-free antioxidant additive.

Also advantageously, the lubricating composition of the invention comprises from 0.5 to 2% by weight of at least one antioxidant additive relative to the total weight of the composition.

The lubricating composition of the invention may also comprise at least one detergent additive.

Detergent additives generally allow a reduction in the formation of deposits on the surface of metal parts by dissolving secondary oxidation and combustion products.

The detergent additives that can be used in the lubricating composition of the invention are generally known to persons skilled in the art.

The detergent additives may be anionic compounds comprising a long lipophilic hydrocarbon chain and hydrophilic head. The associated cation may be a metal cation of an alkaline or alkaline-earth metal.

The detergent additives are preferably selected from among the salts of alkaline metals or alkaline-earth metals of carboxylic acids, sulfonates, salicylates, naphthenates, and phenate salts. The alkaline and alkaline-earth metals are preferably calcium, magnesium, sodium or barium.

These metal salts generally comprise the metal in stoichiometric amount or in excess i.e. an amount greater than the stoichiometric amount. They are then overbased detergent additives; the excess metal imparting the overbased nature to the detergent additive is then generally in the form of an oil-insoluble metal salt e.g. a carbonate, hydroxide, an oxalate, acetate, glutamate, preferably a carbonate.

Advantageously, the lubricating composition of the invention may comprise from 2 to 4% by weight of detergent additive relative to the total weight of the lubricating composition.

Also advantageously, the lubricating composition of the invention may further comprise at least one pour point depressant additive.

By slowing the formation of paraffin crystals, pour point depressants generally improve the behaviour of the lubricating composition of the invention under cold temperatures.

As examples of pour point depressant additives, mention can be made of alkyl polymethacrylates, polyacrylates, polyarylamides, polyalkylphenols, polyalkylnaphthalenes, alkylated polystyrenes.

Advantageously the lubricating composition of the invention may also comprise at least one dispersant.

The dispersant can be selected from among Mannich bases, succinimides and derivatives thereof.

Also advantageously, the lubricating composition of the invention may comprise from 0.2% to 10% by weight of dispersant relative to the total weight of the lubricating composition.

The lubricating composition of the present invention may also comprise at least one additive improving the viscosity index. As examples of additives improving the viscosity index, mention can be made of polymer esters, hydrogenated or non-hydrogenated homopolymers or copolymers of styrene, butadiene and isoprene, polyacrylates, polymethacrylates (PMAs) or olefin copolymers in particular ethylene/propylene copolymers.

The lubricating composition of the invention may be in different forms. The lubricating composition of the invention may particularly be an anhydrous composition.

Preferably, this lubricating composition is not an emulsion.

The above-defined lubricating composition is used to prevent or reduce pre-ignition in an engine.

By engine according to the invention is more particularly meant vehicle engines such as:

-   -   engines of motor vehicles including petrol engines and diesel         engines, but also engines powered by gas and petrol (bifuel         gas/petrol engines) and engines powered by gas and diesel oil         (bifuel gas/diesel oil engines);     -   engines of heavy vehicles, and more specifically heavy vehicle         engines powered by gas.

By engine according to the invention is also meant 4-stroke engines and more specifically marine 4-stroke engines, preferably marine 4-stroke engines powered by gas.

In one preferred embodiment of the invention, the lubricating composition is used to prevent or reduce pre-ignition in a vehicle engine, preferably a motor vehicle engine.

Pre-ignition, according to the invention, includes the phenomenon of low frequency vibration producing a rumble sound.

By pre-ignition according to the invention is more particularly meant low speed pre-ignition (LSPI).

In one preferred embodiment of the invention, the lubricating composition is used to prevent or reduce low speed pre-ignition (LSPI) in an engine, preferably of a vehicle, advantageously of a motor vehicle.

A further subject of the invention is a method for preventing or reducing pre-ignition in an engine, said method comprising at least one step to place a mechanical part of the engine in contact with a lubricating composition such as defined above.

All the characteristics and preferences described for the above use also apply to this method.

A further subject of the invention is the use of a polyalkylene glycol (PAG) obtained by polymerisation or copolymerisation of alkylene oxides having 3 to 8 carbon atoms, including at least one butylene oxide, in a lubricating composition comprising at least one base oil to prevent or reduce pre-ignition in an engine.

All the characteristics and preferences relating to the PAG and the lubricating composition described above also apply to this use. 

1-9. (canceled)
 10. Method to prevent or reduce pre-ignition in an engine comprising the lubrifiation of said engine with a lubricating composition comprising at least one base oil and at least polyalkylene glycol (PAG) obtained by polymerisation or copolymerisation of alkylene oxides having 3 to 8 carbon atoms, including at least one butylene oxide.
 11. The method according to claim 10, wherein the PAG is a PAG of formula (A):

where: Y₁ and Y₂ are each independently a hydrogen atom or hydrocarbon group; n is an integer of 2 or higher; x represents one or more integers ranging from 1 to n; the groups R_(2x-1) and R_(2x) are each independently a hydrogen atom or hydrocarbon group having 1 to 6 carbon atoms, the sum of the number of carbon atoms of R_(2x-1) and R_(2x) is equal to 2 for at least one value of x.
 12. The method according to claim 10, wherein the PAG is a PAG of formula (A) where: Y₂ is a hydrogen atom; R_(2x-1) and R_(2x), the same or different, are independently a hydrogen atom or C₁-C₂-alkyl group.
 13. The method according to claim 10, wherein the PAG is a PAG of formula (B):

where: R1 is a straight-chain or branched C₈-C₁₂-alkyl group; p is an integer ranging from 2 to 10; q is an integer ranging from 2 to
 10. 14. The method according to claim 10, wherein the weight content of PAG, relative to the total weight of the composition, ranges from 5 to 50%.
 15. The method according to claim 10, wherein the lubricating composition also comprises an additive selected from among from among friction modifiers, detergents, anti-wear additives, extreme-pressure additives, viscosity index improvers, dispersants, antioxidants, pour point improvers, defoamers, thickeners and mixtures thereof.
 16. The method according to claim 10 wherein the engine is a vehicle engine.
 17. The method according to claim 10, wherein the pre-ignition is low speed pre-ignition (LSPI) and the engine is a vehicle engine.
 18. The method of claim 11, wherein Y₁ and Y₂ are each a C₁-C₃₀ alkyl or alkylphenyl group.
 19. The method of claim 18, wherein n is less than
 500. 20. The method of claim 18, wherein n is in a range of 2 to
 200. 21. The method of claim 18, wherein n is in a range of 2 to
 60. 22. The method of claim 18, wherein n is in a range of 5 to
 30. 23. The method of claim 18, wherein n is in a range of 7 to
 20. 24. The method of claim 18, wherein n is in a range of 7 to
 15. 25. The method of claim 18, wherein the groups R2x-1 and R2x are each independently a straight-chain or branched C1-C6 alkyl group.
 26. The method of claim 13, wherein p is an integer ranging from 2 to
 6. 27. The method of claim 13, wherein q is an integer ranging from 2 to
 6. 28. The method of claim 14, wherein the weight content of PAG, relative to the total weight of the composition, ranges from 10 to 40%.
 29. The method of claim 14, wherein the weight content of PAG, relative to the total weight of the composition, ranges from 15 to 30%. 